Power management in data centers has become an increasingly important concern. Large server installations are designed to handle peak load, which may be significantly larger than in off-peak conditions. The increasing cost of energy consumption and cooling incurred in farms of high-performance web servers make low-power operation during off-peak hours desirable. This paper investigates adaptive algorithms for dynamic voltage scaling in QoS-enabled web servers to minimize energy consumption subject to service delay constraints. We implement these algorithms inside the Linux kernel. The instrumented kernel supports multiple client classes with per-class deadlines. Energy consumption is minimized by using a feedback loop that regulates frequency and voltage levels to keep the synthetic utilization 1 around the aperiodic schedulability bound derived in an earlier publication. Enforcing the bound ensures that deadlines are met. Our evaluation of an Apache server running on the modified Linux kernel shows that non-trivial off-peak energy savings are possible without sacrificing timeliness.